There are many applications in which it is desired to measure acceleration, and devices for measuring acceleration are referred to as accelerometers. Because accelerometers are often used in conjunction with integrated circuitry, it is sensible to manufacture accelerometers in manners consistent with integrated circuitry fabrication techniques. Accordingly, many accelerometer designs have been disclosed in the art, and which are basically fabricated as microelectromechanical systems (MEMS) using essentially standard semiconductor manufacturing techniques, such as those based on silicon substrates.
In an accelerometer, a moveable mass is fabricated, and although the mass should be significant, it is in actuality quite small given the semiconductor processing techniques that are used to form it. The mass is sensitive to acceleration, such that an acceleration places a force on the mass (i.e., F=ma). The force generally causes the mass to move, and this movement is somehow electronically detected. For instance, the mass might define a first plate of a capacitor and its movement may vary the spacing between that first plate and a fixed second capacitor plate. A change in plate spacing scales with the inverse of capacitance, as is well known. Thus, as the mass moves, the capacitance of the capacitor will change, and this change in capacitance can be detected and correlated to the acceleration.
While an accelerometer measures the absolute value of the acceleration, an acceleration switch either turns on or off depending on the amount of acceleration that is present. Such acceleration switches are generally designed to have an acceleration threshold that determines the tripping point of the switch. For example, if the acceleration is less than 30 Gs, the switch will be open and will not conduct current; if greater than 30 Gs, the switch will be closed and will conduct a current. In any event, such acceleration switches are generally built using the same principles as are accelerometers. Continuing the example above, if the mass is designed to move such that the first plate touches the fixed second plate at an acceleration of 30 G, then a switch as just described is fabricated. In these examples, the only differences between the accelerometer and the acceleration switch are whether their plates touch during a useful operating range and the detection circuitry to be used (capacitance v. current). Other examples of accelerometers of acceleration switches can be found in the following references, all of which are hereby incorporated by reference in their entireties: U.S. Pat. Nos. 5,545,912; 6,074,890; 7,009,124; and 6,984,541.
One application in which an accelerometer switch is useful is in a wireless tire pressure sensor. Because wireless tire pressure sensors are known, they are not discussed in great detail, and instead the reader is referred to the following U.S. patent applications, which are hereby incorporated by reference in their entireties: U.S. Ser. No. 11/144,549 filed on Jun. 3, 2005 and U.S. Ser. No. 11/144,992 both filed on Jun. 3, 2005. Briefly, such tire pressure sensors generally are insertable into a tire rim's valve stem and include an electronic pressure sensor for measuring the tire pressure, a radio frequency (RF) transmitter, and a power source such as a battery. The tire pressure is read by the sensor and the resulting pressure data is sent to a RF transmitter for transmission to the vehicle's computer, which in turn provides a proper indication of tire pressure to the driver (e.g., such as by illuminating a low tire indicator on the vehicle's dashboard).
An acceleration switch can be used in the context of a wireless tire pressure sensor to conserve battery power. In this regard, it is desirable to more frequently transmit tire pressure data from the sensor to the vehicle's computer at higher vehicle speeds than at lower speeds. Thus, it might be preferred to transmit tire pressure data more frequently, perhaps every 30 seconds, when the car is traveling faster than 25 mph. However, it might only be necessary to transmit tire pressure data quite infrequently when the car is traveling slower than 25 mph or not moving at all; in such cases, transmitting at the faster rate would be wasteful of battery power. Accordingly, the accelerometer switch can be tuned to trip at 25 mph. (Note that the centripetal acceleration detected by the acceleration switch scales with the square of the rotational speed divided by the radius from the center of the wheel). Therefore, the acceleration switch can be used to control the tire pressure data transmission rate, saving battery life.
While both accelerometers and accelerator switches are useful, previous designs have been complicated to manufacture, or suffer from reliability concerns. For example, in accelerations switches, which rely on an electrode on the moving mass making contact with another electrode to establish a current, the mechanical action at the electrodes can cause them to degrade. Or, the passage of current through the electrodes may cause localized resistive heating which cause the electrodes to weld together. This means currents lower than optimal might have to be used. In short, an improved accelerometer or acceleration switch design that is reliable and cheaper to produce would be of benefit to the art of tire pressure sensors, and any other arts in which such devices could be used.